The present invention relates to machines for applying components of articles of hardware to sheet- or web-like workpieces, e.g., for applying rivets or like articles to jeans, jackets or other garments. Typical examples of such machines are riveting presses wherein a stationary lower tool is disposed in register with a vertically reciprocable upper tool. A component of an article of hardware, which is placed between one of the tools and a workpiece which is also disposed between the tools is automatically attached to the workpiece when the reciprocable tool descends. If the article of hardware comprises two components which must be attached to one another as well as to the workpiece, one of the components is placed onto the lower tool below the workpiece and the other component is placed in front of the descending upper tool at a lever above the workpiece so that one of the components penetrates through the workpiece and is attached to the other component in response to last stage of downward movement of the upper tool.
As a rule, the component which is placed in front of the descending upper tool is held by suitable grippers which are movable up and down with and relative to the upper tool. The upper tool is reciprocated by a rotary cam, and a discrete second cam can be provided to move the grippers toward and away from the upper side of the workpiece, not unlike the hold-down device in a sewing machine is movable relative to the needle.
German Offenlegungsschrift No. 29 15 328 discloses a riveting press wherein the grippers receive motion from their cam by way of two motion transmitting members which are movable relative to one another and constitute elements of a safety device serving to prevent injury to a careless or inexperienced operator. The upper motion transmitting member carries at its upper end a follower for the respective cam and the lower motion transmitting member carries the grippers at its lower end. The electric motor which drives the cam for the grippers is arrested in response to opening of an electric switch whose contacts are mounted on the two motion transmitting members in such a way that the switch opens in automatic response to shifting of the upper member relative to the lower member when the grippers on the lower member encounter an obstruction (e.g., one or more fingers) on their way toward the upper side of the workpiece between the two tools. A spring biases the motion transmitting members in a direction to increase the distance between the follower on the upper member and the grippers on the lower member, and such spring yields when the downward movement of the lower member is terminated prematurely by an obstruction in the path of downward movement of the grippers. The roller follower of the upper motion transmitting member extends into an endless groove of the respective cam so that it is compelled to move between predetermined upper and lower end positions in response to each revolution of the cam. This ensures that the upper member moves relative to the lower member, against the opposition of the aforementioned spring, when the lower member and its grippers are held against further or any downward movement with the upper member due to the presence of an obstruction in the path of movement of the grippers toward the upper side of the workpiece.
The aforementioned German printed publication further discloses an auxiliary cam which is designed to engage the lower motion transmitting member when the latter approaches or reaches the lower end of its downward stroke to thus hold the lower member in the lower end position. This auxiliary cam is intended to prevent unintentional opening of the switch in the circuit of the electric motor under the action of a compressed workpiece which tends to expand and to move the grippers and the lower motion transmitting member upwardly which, in the absence of the auxiliary cam, would result in immediate stoppage of the motor as a consequence of movement of the upper member relative to the lower member. The provision of auxiliary cam contributes to the complexity, cost and bulk of the press which is disclosed in the German printed publication. Moreover, the provision of the auxiliary cam reduces the versatility of the press because it limits the extent to which the thicknesses of the workpieces to be treated in the machine can deviate from an optimum or average thickness. In other words, if a workpiece is relatively thick, the lower motion transmitting member cannot reach that (lower end) position in which it can be engaged and held by the auxiliary cam unless it is urged downwardly with a force which is likely to cause breakage of machine parts and/or permanent damage to a relatively thick workpiece. On the other hand, pronounced versatility of a riveting press is often not only desirable but absolutely necessary, e.g., when the machine is to apply components of articles of hardware to portions of garments (such as jeans or jackets) wherein one or more first articles of hardware must be applied to a single layer of textile material but one or more second articles of hardware must be applied to a portion of a garment which consists of a substantial number of layers so that its thickness is a multiple of the thickness of a single layer. It is not unusual to apply articles of metallic or plastic hardware to a piece of garment which consists of as many as six or even more layers. The aforediscussed riveting press which is disclosed in the German printed publication does not exhibit the required versatility for such types of operations, except if one would discard the safety feature which is unacceptable to most manufacturers for obvious reasons.